3 -Phenylpyrazoles, tautomerism

The study of tautomerism using H NMR spectroscopy is simple when the tautomers give separate signals (84B2906) otherwise, interpolation methods need to be applied, which entail several sources of imprecision [83JPR(325)238]. A paper reports the observation of two NH signals for the N-labeled tautomers of 3(5)-methyl-5(3)-phenylpyrazole (45) in toluene-dg at 190 K (Scheme 15) [92JCS(P2)1737]. 

Biquard and Grammaticakis demonstrated in 1941 that the ultraviolet spectra of 3-methyl- and 3,4-dimethyl-l-phenylpyrazol-5-one were intermediate between those of methylated derivatives of the CH (i.e., 55, R = R — Me) and NH forms (i.e., 56, R = Me, R — H or Me) of the compounds, concluding that the potentially tautomeric compounds exist as mixtures of these forms. However, they did not consider the possible occurrence of an OH form. Other ultraviolet... 

Italian investigators" have discussed the tautomerism of 5-amino-3-methyl-l-phenylpyrazole in terms of structures 183 and 184 (185 was not considered) and appear to be of the opinion that reaction... 

Bensaude et al. (78T2259) have used T-jump relaxation spectrophotometry to determine the rates of protonation and deprotonation of 3(5)-methyl-5(3)-phenylpyrazole anion (416") and cation (416H ), respectively. This study is a fundamental cornerstone in understanding annular tautomerism in azoles. The nondissociative intramolecular proton transfer in azoles is not observed (78T2259 86BSF429). 

A comprehensive report has described the nitration of 3-hydroxy-1-phenyl pyrazoles [75JCS(P2)1609]. These hydroxy compounds are tautomeric with pyrazolones but at equilibrium the hydroxy form is favored. 3-Hydroxy-5-methyl-l-phenylpyrazole is nitrated in acetone using pentyl... 

FVP experiments were carried out between 500 and 800 °G on 3(5)-phenyl- and 3(5)-methylpyrazolinones and on 3(5)-methoxy-5(3)-phenylpyrazole < 1999J(P2)211 >. The origin of the isolated products (mainly indanone, hydroxy-alkynes, and a,/ -unsaturated aldehydes) were explained as arising from the hydroxy tautomers of pyrazolinones. Temperature effects on the tautomeric equilibrium of 1-phenyl-3-methylpyrazolinone in solution show that the percentage of the GFI tautomer increased with increasing temperature. 

One of the most significative changes in H NMR spectroscopy of iVH-pyrazoles, is the systematic use of low temperature to block the proton transfer and, thus, determine by simple integration of signals the tautomeric equilibrium constant. In this way, the Kt values of 3(5)-phenylpyrazole (57) <9iG477> and 3(5)-phenyl-5(3)-methylpyrazole (64) <92JCS(P2)1737> were determined (see Section... 

The last paper <91G477> deals more specifically with the problem of tautomerism of 3(5)-phenyl-pyrazole (57). By the use of UV spectroscopy, semiempirical calculations (CNDO/S//CNDO/2) and low temperature C NMR spectroscopy the tautomeric equilibrium (80% (57a), 20% (57b) was determined. One of the most important conclusions of this work, as far as electronic spectra are concerned, is that the absorption of phenylpyrazoles is much more dependent on the conformation of the phenyl ring than on its position, 3, 4, or 5. 

Cp) 138.3 (C-OH) and those of =NNHC6H5 in 86 were 142.6 (Q) 115.5 (CJ 129.6 (Cm) 124.5 (Cp) 186.1 (C=0). A similar approach was used9 for azo dyes with intramolecular hydrogen bonds. 2-Hydroxy-5-tcr/-butylazobenzene 87 (model azo compound) and 3-methyl-l-phenylpyrazole-4,5-dione 4-phenyl-hydrazone 78 (hydrazone model) and compound 48, as a representative of classical tautomeric system, werestudied. The 13Cchemicalshiftsofthephenylgroupsenable an estimation of hydrazone content from chemical shifts of Q, Cc and Cp to be 66%, 61% and 66%, respectively. 

In a study of the tautomerism of 3-(5)-phenylpyrazoles, the X-ray structure of 4-bromo-3-phenylpyrazole showed that a trimeric structure bonded through an H-bond was formed with a considerable twist of the phenyl group about the pyrazole-phenyl bond (92JCS(P2)1737). 

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